Bacteria respond to many external stresses through two-component systems (TCSs). Some M. tuberculosis TCSs influence virulence, but little is known about the specific stresses to which they respond. The M. tuberculosis mprAB TCS includes the putative transcription factor, mprA. Our data indicate that mprA is activated by conditions which can damage the cell envelope, including exposure to the chemical surfactant SDS. Our results show that mprA is autoregulatory and that, under SDS stress, it has a major role in activating the envelope stress response through sigma factor E. We hypothesize that mprA may also respond to the respiratory surfactant, surfactant protein A (SP-A), as recent evidence indicates that SP-A increases the membrane permeability of Gram-negative bacteria and inhibits their growth, and our preliminary evidence indicates that SP-A also directly inhibits the growth of M. tuberculosis. Our specific aims are to 1) confirm that mprA functions as a transcriptional activator. Expression from the mprA and sigE promoters will be examined using lacZ reporter constructs in parental strains and mprA deletion mutants. Binding of mprA to promoters will be studied in vitro by gel-shift analyses, and in live bacteria, using nucleoid immunoprecipitation, in which mprA/DNA complexes will be recovered using an antibody against epitope-tagged mprA; 2) evaluate the role of mprA during surfactant exposure. Growth of M. tuberculosis parental and mprA mutant strains will be compared in the presence of SP-A and SDS. Use of fluorescent live/dead stains and other methods will be used to assess whether SP-A has a bacteriocidal or bacteriostatic effect on M. tuberculosis parental strains and mprA mutants. SP-A effects on the expression of mprA and other genes will be evaluated by real-time PCR. These studies will enhance our understanding of the role of mprA in gene expression and survival, and will provide a foundation for defining the direct effects of lung surfactants on this major respiratory pathogen.